Torque sensing apparatus

ABSTRACT

There is described an apparatus for providing rotary displacement information indicative of the torque applied to a torsion bar which is rotatable relative to a housing, in which the electromagnetic coupling between a transmit aerial fixed relative to the housing and a receive aerial fixed relative to the housing varies in dependence upon first and second resonators fixed to the torsion bar at spaced axial positions. The first and second resonators have respective different resonant frequencies to enable the signal coupled between the transmit aerial and the receive aerial via the first resonator to be distinguished from the signal coupled between the transmit aerial and the receive aerial via the second resonator.

This invention relates to the sensing of position or speed, and hasparticular, but not exclusive, relevance to a system for measuringtorque and component parts thereof.

Torque measuring systems are employed, for example, in automobiles formeasuring the torque applied to rotating members such as the steeringwheel. In order to measure torque, relative rotary displacement betweentwo points along the axis of rotation of a torsion bar is measured.

Inductive sensors have been used in the past for non-contact positionmeasurement. The present invention addresses techniques for theincorporation of inductive position sensing in a torque sensor.

According to an aspect of the present invention, there is providedapparatus for generating rotary displacement information indicative ofthe torsion applied to a torsion bar which is rotatable relative to ahousing, in which the electromagnetic coupling between a transmit aerialfixed relative to the housing and a receive aerial fixed relative to thehousing varies in dependence upon first and second resonators fixedrelative to respective spaced axial positions of the torsion bar. Thefirst and second resonators have respective different resonantfrequencies to enable the signal induced in the receive aerial by thefirst resonator to be distinguished from the signal induced in thereceive aerial by the second resonator.

An embodiment of the present invention will now be described withreference to the attached figures in which:

FIG. 1 schematically shows a sectional view of a coupling arrangementbetween a steering wheel and a gear of a rack-and-pinion type steeringmechanism;

FIG. 2 schematically shows the main components of a position sensorforming part of the coupling arrangement illustrated in FIG. 1;

FIG. 3 shows a perspective view of a sleeve member, having mountedthereon a flexible printed circuit board, forming part of the couplingarrangement illustrated in FIG. 1;

FIG. 4 shows a plan view of the flexible printed circuit boardillustrated in FIG. 3 when laid out flat;

FIG. 5 shows a perspective view of a first puck wheel, having mountedthereon a flexible printed circuit board, forming part of the couplingarrangement illustrated in FIG. 1;

FIG. 6 shows a plan view of the first puck wheel illustrated in FIG. 1;

FIG. 7 shows a plan view of the flexible printed circuit boardillustrated in FIG. 5 when laid out flat;

FIG. 8 shows a perspective view of a second puck wheel, having mountedthereon a flexible printed circuit board, forming part of the couplingarrangement illustrated in FIG. 1;

FIG. 9 shows a plan view of the flexible printed circuit boardillustrated in FIG. 8 when laid out flat;

FIG. 10 schematically shows a perspective view of the positionalrelationship between the first and second puck wheels when mounted inthe coupling arrangement illustrated in FIG. 1;

FIG. 11 schematically shows an exploded view of the positionalrelationship between the sleeve, the first puck wheel and the secondpuck wheel;

FIG. 12 shows a perspective view of the positional relationship betweenthe sleeve, the first puck wheel and the second puck wheel when mountedin the coupling arrangement illustrated in FIG. 1;

FIG. 13 schematically shows the main components of an ASIC forming partof the position sensor illustrated in FIG. 11;

FIG. 14 is a graph showing the relationship between a first detectedphase angle and the absolute position of a first shaft forming part ofthe coupling arrangement illustrated in FIG. 1; and

FIG. 15 is a graph showing the relationship between a second detectedphase angle and the absolute position of the first shaft forming part ofthe coupling arrangement illustrated in FIG. 1.

In the illustrated embodiment of the invention, a car has a steeringwheel which is connected to a gear forming part of a rack-and-pinionsteering mechanism. FIG. 1 shows a cross-sectional view of a couplingarrangement between the steering wheel and the gear.

A first elongate cylindrical shaft 1 is attached at one longitudinal end5 to the steering wheel (not shown). As shown in FIG. 1, the first shaft1 has a reduced diameter axial portion 3 extending from the longitudinalend 7 away from the steering wheel to a stepped region 9. A secondelongate cylindrical shaft 11 is attached at one longitudinal end 13 tothe gear (not shown) of the rack-and-pinion steering mechanism, and hasa hollow axial portion 15 extending from the longitudinal end 17 awayfrom the gear, hereafter called the open end 17. As shown in FIG. 1, thereduced diameter portion 3 of the first shaft 1 is mounted in the hollowportion 15 of the second shaft 11 with the first shaft 1 and the secondshaft 11 axially aligned and the stepped region 9 of the first shaft 1being adjacent the open end 17 of the second shaft 11. A locking pin 19fixes the first shaft 1 to the second shaft 11 towards the end 7 of thereduced diameter portion 3.

For the remainder of this specification, the term axial direction refersto the direction of the common longitudinal axis of the first and secondshafts 1 and 11, the term radial direction refers to lines radiatingperpendicularly away from the axial direction, and the termcircumferential direction refers to a direction normal to both the axialdirection and the radial direction.

The first shaft 1 and the second shaft 11 are rotatably mounted relativeto a housing 21, so that when a driver of the car turns the steeringwheel both the first shaft 1 and the second shaft 11 rotate relative tothe housing 21. In particular, in this embodiment the range ofrotational movement of the first and second shafts is two fullrevolutions, i.e. 720°, relative to the housing 21.

In this embodiment, the steering mechanism is an electronicpower-assisted steering mechanism in which electrical motors apply anassisting force which varies in dependence on the torque applied to thesteering wheel by the driver. Accordingly, the torque applied by thedriver must be monitored.

The torque applied to the steering wheel by the driver is transferred tothe gear via the locking pin 19 which fixes the first shaft 1 to thesecond shaft 11. However, the axial distance between the locking pin 19and the junction between the stepped region 9 of the first shaft 1 andthe open end 17 of the second shaft 11 results in a relative rotarydisplacement between the stepped region 9 and the open end 17 whichvaries in dependence on the applied torque. According to the invention,an inductive sensor measures the relative rotary displacement betweenthe stepped region 9 and the open end 17, and the applied torque iscalculated from the measured relative rotary displacement.

The inductive sensor of the present invention has: an aerial member 23which is mounted on an aerial guide 25 which is fixed relative to thehousing 21; a first intermediate coupling element 27 (not shown inFIG. 1) which is mounted on a first sleeve member 29 which is fixedrelative to the first shaft 1; and a second intermediate couplingelement 31 which is mounted on a second sleeve member 33 which is fixedrelative to the second shaft 11. The aerial member 23 has formed thereona transmit aerial (not shown in FIG. 1) which generates a magnetic fieldwhich varies around the circumference of the first shaft 1 and thesecond shaft 11, and a receive aerial (not shown in FIG. 1). Thetransmit aerial is balanced relative to the receive aerial so that inthe absence of the first intermediate coupling element 27 and the secondintermediate coupling element 31 no nett signal would be induced in thereceive aerial by virtue of the magnetic field generated by the transmitaerial, but in the presence of the first intermediate coupling element27 and the second intermediate coupling element 31 a signal is inducedin the receive aerial which depends on the rotary positions of the firstshaft 1 and the second shaft 11.

In this embodiment, for improved safety the inductive sensor has twoindependent sensing arrangements providing respective readings for therelative rotary displacement of the first shaft 1 and the second shaft11. In this way, if one sensing arrangement fails a measurement of thetorque may still be calculated using the relative rotary displacementreading provided by the other sensing arrangement. As shown in FIG. 1,the inductive sensor has two ASICs 35 a, 35 b, with each ASIC 35 beingused by a respective different one of the two sensing arrangements.

FIG. 2 schematically shows the main components of the torque sensingcircuitry. In FIG. 2, the first sensing arrangement 41 a and the secondsensing arrangement 41 b are schematically indicated by dashed boxes.

Each independent sensing arrangement 41 has two associated excitationwindings 43 (performing the transmit aerial function for that sensingarrangement) and one sensor winding 45 (performing the receive aerialfunction for that sensing arrangement). In particular, the first sensingarrangement 41 has first and second excitation windings 43 a and 43 band a first sensor winding 45 a which are formed on the aerial member 23and are connected to the first ASIC 35 a. The first sensing arrangementalso has a first resonant circuit 47 a which is formed on the firstintermediate coupling element 27 and a second resonant circuit 47 bwhich is formed on the second intermediate coupling element 31.Similarly, the second sensing arrangement 41 b has third and fourthexcitation windings 43 c and 43 d and a second sensor winding 45 b whichare formed on the aerial member 23 and are connected to the second ASIC35 b, a third resonant circuit 47 c which is formed on the firstintermediate coupling element 27 and a fourth resonant circuit 47 dwhich is formed on the second intermediate coupling element 31.

In this embodiment, for the first sensing arrangement 41 a the first andsecond excitation windings produce radial magnetic field componentswhich vary through twenty cycles of the sine and cosine functionsrespectively around a full circumference, and the third and fourthexcitation windings produce radial magnetic field components which varythrough nineteen cycles of the sine and cosine functions respectivelyaround a full circumference. The radial magnetic field components inducea signal in the first resonant circuit 47 a which varies in accordancewith the rotary position of the first shaft 1, and induce a signal inthe second resonant circuit which varies in accordance with the rotaryposition of the second shaft 11. The signals induced in the first andsecond resonant circuits 47 a, 47 b induce corresponding signalcomponents in the first sensor winding 45 a which are processed by thefirst ASIC 35 a to determine the relative rotary displacement betweenthe first shaft 1 and the second shaft 11. The second sensingarrangement 41 b works in an analogous manner.

The ASIC 35 of each sensing arrangement outputs the calculated relativerotary displacement to a central control unit 49 of the car, whichprocesses the relative rotary displacements to calculate the torqueapplied to the steering wheel.

FIG. 3 schematically shows a perspective view of the aerial member 23and the aerial guide 25. As shown, the aerial guide 25 is a cylindricalsleeve having an outer surface with a recessed circumferential portionin which the aerial member 23 is fixedly mounted. In this embodiment,the aerial member 23 is a rectangular sheet of two-layer flexibleprinted circuit board (PCB) material having a length which is longerthan the circumference of the recessed portion of the aerial guide 25.The aerial member 23 has conductive tracks deposited on either sidewhich are connected, using via holes, to form the excitation windings 43and the sensor windings 45. Two sets of six electrical contacts 51 areprovided, with each set of six electrical contacts 51 connecting the twoexcitation windings and the sensor winding of a sensing arrangement withthe corresponding ASIC 35.

FIG. 4 is a schematic plan view of the aerial member 23 when laid outflat, in which the conductive tracks formed on one side of the PCB arerepresented by solid lines whereas the conductive tracks formed on theother side of the PCB are represented by dashed lines. As shown in FIG.3, the conductive tracks 61 associated with the first sensingarrangement 41 a are spaced apart from the conductive tracks 63associated with the second sensing arrangement 41 b in the widthwisedirection of the PCB (which corresponds to the axial direction when theaerial member 23 is mounted on the aerial guide 25).

In this embodiment, the excitation windings 43 and the sensor winding 45for each sensing arrangement 41 include planar coil arrangements whichextend over a length 65 of the PCB corresponding to the circumference ofthe recessed portion of the aerial guide 25. The excitation windingsproduce magnetic fields having a magnetic field component perpendicularto the PCB which varies in accordance with multiple periods of the sinefunction and the cosine function respectively in substantially the samemanner as the excitation windings described in UK Patent Application GB2374424A (the whole contents of which are hereby incorporated herein byreference). Further, in this embodiment the sensor winding of a sensingarrangement is formed by a multi-loop planar coil extending around thewhole of the length 61.

FIG. 5 schematically shows a perspective view of the first intermediatecoupling element 27 and the first sleeve member 29. As shown, the firstsleeve member 29 has a cylindrical recess 71 for receiving the firstshaft 1. The first sleeve member 29 also has a guide portion 73 on whichthe first intermediate coupling element 27 is mounted. The guide portion73 has two opposing arc portions 75 a and 75 b, which are centred on theaxis of the cylindrical recess 71, and two opposing connecting portions77 a and 77 b which interconnect the two arc portions 75 a, 75 b.

FIG. 6 shows a plan view of the first sleeve member 29 (i.e. lookingalong the axial direction when mounted to the first shaft 1) showing thecylindrical recess 71 and the guide portion 73. As shown, the opposingarc portions 75 a, 75 b each extend approximately 70° around thecylindrical recess 71, and the connecting portions 77 a, 77 b extendinside of the circle of which the outer surfaces of the two arc portions75 form part of the circumference.

Returning to FIG. 5, the two arc portions 75 include projecting partscausing the arc portions 75 to have approximately twice the axial extentof the two connecting portions 77. In this way, the side of the firstsleeve member 29 including the projecting parts has a castellatedappearance.

The first intermediate coupling arrangement includes a two-layerflexible PCB 79 having conductive tracks deposited on either side whichare interconnected by via holes to form the inductors for the first andthird resonant circuits 47 a, 47 c. FIG. 7 shows a schematic plan viewof the flexible PCB 79 when laid out flat, with conductive tracks on oneside of the PCB 79 being represented by solid lines and conductivetracks on the other side of the PCB 79 being represented by dashedlines.

As shown in FIG. 7, the PCB 79 has two end parts 91 a, 91 b, whosedimensions match the dimensions of respective arc portions 75 of theguide portion 73, and a connecting part 93 which is of reduced width andinterconnects the two end parts 91. The connecting part 93 separates thetwo end parts 91 by a distance which allows the two end parts 91 to bemounted to the outer surface of the two arc portions 75 of the guideportion 73, with the connecting part 93 itself being mounted to one ofthe connecting portions 77 of the guide portion 73 (as shown in FIG. 4).

The inductor for the first resonant circuit 47 a is formed by the serialconnection of eight periodically-spaced current loop structures 95 a-95h and the inductor for the third resonant circuit 47 c is formed by theserial connection of seven periodically-spaced current loop structures97 a-97 g. The current loop structures 95, 97 are arranged so that, whenmounted to the sleeve member 29 as shown in FIG. 5, the current loops 95for the first resonant circuit 47 a are spaced apart in the axialdirection from the current loops 97 of the third resonant circuit 47 c.In particular, the axial spacing between the current loops for the firstand third resonant circuits 47 a, 47 c equals the axial spacing betweenthe excitation/sensor windings for the first sensing arrangement 41 aand the excitation/sensor windings for the second sensing arrangement 41b.

As shown in FIG. 7, the current loop structures 95,97 are formed in theend parts 91 of the flexible PCB 79. Two terminals 99 a and 99 b areformed in the connecting part 93 of the flexible PCB 79 to which acapacitor (not shown) is mounted to form the first resonant circuit 47 awith the inductor formed by the current loop structures 95, and twoterminals 101 a and 101 b are formed in the connecting part 93 of theflexible PCB 79 to which a capacitor is mounted to form the thirdresonant circuit 47 c with the inductor formed by the current loopstructures 97. The current loop structures 95 for the first resonantcircuit 47 a are mounted adjacent the projecting parts of the firstsleeve member 29.

In this embodiment, the first resonant circuit has a resonant frequencyof 3.75 MHz and the third resonant circuit has a resonant frequency of 5MHz. Further, the periodic spacing of the current loop structures 95 forthe first resonant circuit 47 a corresponds to an angular spacing of 18°(that is 360° divided by twenty), and the period spacing of the currentloop structures 97 for the third resonant circuit 47 c corresponds to anangular spacing of 18.95° (that is 360° divided by nineteen).

FIG. 8 schematically shows a perspective view of the second intermediatecoupling element 31 and the second sleeve member 33. As shown, thesecond sleeve member 33 has a substantially identical castellated shapeto the first sleeve member 29.

The second intermediate coupling element 33 is formed by a two-layerflexible PCB 111 in a similar manner to the first intermediate couplingelement 29. FIG. 9 schematically shows a plan view of the flexible PCB111 when laid out flat with the conductive tracks on one side beingrepresented by solid lines and the conductive tracks on the other sidebeing represented by dashed lines. Eight current loop structures 113 ato 113 h in the end portions of the flexible PCB 111 are connected inseries with a capacitor (not shown) connected between terminals 115 aand 115 b to form the second resonant circuit 47 b, which in thisembodiment has a resonant frequency of 1.875 MHz, and seven current loopstructures 117 a to 117 g in the end portions of the flexible PCB 111are connected in series with a capacitor (not shown) connected betweenterminals 119 a and 119 b to form the fourth resonant circuit 47 d,which in this embodiment has a resonant frequency of 2.5 MHz.

The periodic spacing of the current loop structures 113 for the secondresonant circuit 47 b corresponds to an angular spacing of 18° (that is360° divided by twenty), and the periodic spacing of the current loopstructures 117 for the fourth resonant circuit corresponds to an angularspacing of 18.95° (that is 360° divided by nineteen). The current loopstructures 117 for the fourth resonant circuit 47 d are mounted adjacentthe projecting parts of the second sleeve member 33.

FIG. 10 schematically shows a perspective view of the positionalrelationship between the first intermediate coupling element 27 and thesecond intermediate coupling element 31 when the coupling arrangement isassembled. As shown, the castellated ends of the first and second sleevemembers interlock so that the projecting parts of the arc portions 75 ofone sleeve member are located in the spaces adjacent the outside of theconnecting portions 77 of the other sleeve member. In this way, thecurrent loop structures of the first resonant circuit 47 a are in thesame position along the axial direction as the current loop structuresof the second resonant circuit 47 b but are spaced apart in thecircumferential direction. Similarly, the current loop structures of thethird resonant circuit 47 c are in the same position along the axialdirection as the current loop structures of the fourth resonant circuit47 d but are spaced in the circumferential direction.

FIG. 11 schematically shows an exploded view indicating how the firstsleeve member 29 and the second sleeve member 31 are received in theaerial guide 25, and FIG. 12 shows schematically shows the first sleevemember 29 and the second sleeve member 31 when positioned within theaerial guide 25. When assembled, the first sleeve member 29 and thesecond sleeve member 33 are rotatably mounted within the aerial guide25, with the current loop structures of the first and second resonantcircuits 47 a, 47 b located in the same position along the axialdirection as the first and second excitation windings 43 a, 43 b and thefirst sensor winding 45 a, and the current loop structures of the thirdand fourth resonant circuits 47 c, 47 d located in the same positionalong the axial direction as the third and fourth excitation windings 43c, 43 d and the second sensor winding 45 b.

FIG. 13 schematically shows the main components of the first ASIC 35 a.A first quadrature signal generator 151 a generates a quadrature pair ofsignals which in this embodiment have a frequency of 5 kHz (hereaftercalled the modulation frequency). A second quadrature signal generator151 b generates a quadrature signal at a first carrier frequency whichis equal to the resonant frequency of the first resonant circuit 47 a,which in this embodiment is 3.75 MHz. A third quadrature signalgenerator 151 c generates a quadrature signal at a third carrierfrequency which is equal to the resonant frequency of the secondresonant circuit 47 b, which in this embodiment is 1.875 MHz.

The quadrature pair of signals at the modulation frequency are input toa first modulating arrangement 153 a which modulates the in-phase signalI₁ at the modulation frequency by the in-phase signal at the firstcarrier frequency to generate a signal I₁(t) and modulates thequadrature signal at the modulation frequency by the in-phase signal I₁at the first carrier frequency to generate a signal Q₁(t). Thequadrature pair of signals at the modulation frequency are also input toa second modulating arrangement 153 b which modulates the in-phasesignal at the modulation frequency by the in-phase signal I₂ at thesecond carrier frequency to generate a signal I₂(t) and modulates thequadrature signal at the modulation frequency by the in-phase signal I₂at the second carrier frequency to generate a signal Q₂(t).

The signals I₁(t) and I₂(t) are then input into a first digital mixer155 a which combines the signals I₁(t) and I₂(t), and the resultantcombined signal is amplified by a first coil driver 157 a. The amplifiedsignal output by the first coil driver 157 a is supplied to the firstexcitation winding 43 a. The signals Q₁(t) and Q₂ (t) are input to asecond digital mixer 155 b and the resultant combined signal isamplified by a second coil driver 157 b and supplied to the secondexcitation winding 43 b.

The signal components supplied to the first and second excitationwindings 43 a, 43 b at around the first carrier frequency induce aresonant signal in the first resonant circuit 47 a which varies inaccordance with the radial position of the first shaft 1. The resonantsignal induced in the first resonant circuit 47 a in turn induces asignal in the first sensor winding 45 a. Similarly, the signalcomponents 3Q supplied to the first and second excitation windings 43 a,43 b at around the second carrier frequency induce a resonant signal inthe second resonant circuit 47 b, which in turn induces a signal in thefirst sensor winding 45 a.

As set out in UK Patent Application GB 2374424A, when the signal inducedin the first sensor winding 45 a is input into a first synchronousdetector 159 a which performs synchronous detection using the quadraturesignal Q₁ at the first carrier frequency, the resultant signal output bythe first synchronous detector 159 a has a component at the modulationfrequency whose phase depends on the angular position of the first shaft1. This phase is detected by a first phase detector 161 a. Similarly,when the signal induced in the first sensor winding 45 a is input into asecond synchronous detector 159 b which performs synchronous detectionusing the quadrature signal Q₂ at the second carrier frequency, theresultant signal output by the second synchronous detector 159 b has acomponent at the modulation frequency whose phase depends on the angularposition of the second shaft 11. This phase is detected by a secondphase detector 161 b.

FIG. 14 shows a graph indicating the relationship between the phasedetected by the first phase detector 161 a and the angular position ofthe first shaft 1. As a result of the twenty periods of the excitationwindings over a full revolution, as shown in FIG. 14 the phase detectedby the first phase detector 161 a corresponds to twenty differentabsolute rotary positions of the first shaft 1. Similarly, the phasedetected by the second phase detector 161 b corresponds to twentydifferent rotary positions of the second shaft 11. The first ASIC 35 ais therefore unable to determine the absolute rotary positions of thefirst and second shafts 1,11 (in this specification absolute rotaryposition refers to the rotary position of a shaft relative to areference position; as the shafts can rotate around two full revolutionsthe absolute position does not unambiguously give the position of ashaft within its entire rotary range of movement).

Despite the first and second shafts 1, 11 being able to rotate over arange of approximately 720°, the relative rotary displacement betweenthe first and second shafts 1, 11 is never more than a few degrees,which is well within one period of the readings. A processor 163 istherefore able to calculate and output the relative rotary displacementbetween the first and second shafts 1, 11, thereby allowing the torqueto be calculated by the central control unit 49.

In the second sensing arrangement 41 b, the second ASIC 35 b issubstantially identical to the first ASIC 35 a except that the firstcarrier frequency is set to 5 MHz and the second carrier frequency isset to 2.5 MHz. As discussed, the periodicity of the excitation windingsand the resonant circuits in the second sensing arrangement 41 bcorresponds to nineteen periods over 3600. Therefore, as shown in FIG.15, each phase reading corresponds to nineteen possible rotarypositions. Again, as for the first sensing arrangement 41 a althoughambiguity exists in the absolute position measurement using the secondsensing arrangement 41 b, the range of relative rotary displacementbetween the first shaft 1 and the second shaft 11 is significantly lessthan one period and accordingly the relative rotary displacement may beunambiguously calculated.

In this embodiment, the ASIC 35 of each sensing arrangement 41 outputsthe respective calculated relative rotary displacement to a centralcontrol unit 49 of the car, and also outputs the detected phase anglesto the central control unit 49 of the car. The central control unit 49calculates the torque using the calculated relative rotary displacement.Further, although each individual detected phase angle can not beconverted unambiguously to an absolute position measurement, due to thedifference between the periodicity of the first sensing arrangement 41 aand the second sensing arrangement 41 b the central control unit 49 isable to determine an absolute position measurement using the phasereadings form both sensing arrangements 41 using a Vernier-typecalculation.

The particular arrangement of the excitation windings 43, sensorwindings 45 and resonant circuits 47 of the illustrated embodiment has anumber of advantages. In particular:

-   (1) By using plural periodically-spaced current loop structures in    the resonant circuits the signal induced in each resonant circuit is    increased in comparison with a resonant circuit having a single    current loop structure.-   (2) By arranging the current loop structures in a circumferential    plane with each current loop structure having an opposing current    loop structure, the sensitivity to any ambient electromagnetic field    is reduced. Further, the sensitivity to slight misalignments,    between the axis of rotation of the shafts and the centre of the    circular path of the transmit and receive aerials is reduced. In    addition, such a cylindrical geometry gives an increased tolerance    to axial misalignment between the transmit/receive aerials and the    resonators.-   (3) By circumferentially spacing apart the current loops associated    with each resonator in each sensing arrangement, noise caused by    coupling between adjacent resonant circuits is reduced.-   (4) By axially spacing the excitation windings, sensor winding and    current loop structures of the resonant circuits for the first    sensing arrangement from the excitation windings, sensor winding and    current loop structures of the resonant circuits for the second    sensing arrangement, noise caused by coupling between the sensing    arrangements is reduced.-   (5) By employing the described castellated arrangement, it has been    found that the performance for a given axial distance over which the    inductive sensor extends is improved.

MODIFICATIONS AND FURTHER EMBODIMENTS

As stated above, using plural periodically-spaced current loopstructures in the resonant circuits has the advantage of increasingsignal strength. While in the illustrated embodiment the periodicspacing of the current loop structures matches the period of thecorresponding transmit aerial, the period of the resonant circuits couldbe any integer multiple of the period of the corresponding transmitcoil.

It is not essential to use a plurality of current loop structures ineach resonant circuit, and alternatively each resonant circuit could beformed by a single current loop structure.

In the illustrated embodiment, the excitation windings, the sensorwindings and the current loop structures of the resonant circuits arearranged on circumferential surfaces, leading to advantage (2) above.However, this is not essential and the excitation windings, sensorwindings and current loop structures could, for example, be formed onradial surfaces. In an embodiment, the radial surfaces for the currentloop structures are provided by the surfaces of disks attachedco-axially to the first and second shafts.

While the axial spacing of the sensing arrangements is preferred, it isnot essential. In an alternative embodiment, the current loop structuresfor the first and third resonant circuits (which rotate with the firstshaft) are located on the first shaft at a common axial position and thecurrent loop structures for the second and fourth resonant circuits(which rotate with the second shaft) are located on the second shaft ata common axial position which is spaced axially apart from the commonaxial position of the first and third resonant circuits. In thisalternative embodiment, the transmit aerials and receive aerials for thefirst and second sensing arrangements extend over an axial extentencompassing all the current loop structures. One advantage of such anarrangement is that it does not require interlocking castellatedportions, and accordingly a full range of relative rotary displacementfrom −180° to +180° can be measured. Further, the current loopstructures for resonant circuits formed in the same axial position neednot be circumferentially spaced apart, and in an embodiment the resonantcircuits could be formed by respective series of current loops whichextend entirely around the respective shaft.

In the illustrated embodiment, in each sensing arrangement theassociated resonant circuits are simultaneously energised, and theresultant signals induced in the sensor winding input into parallelprocessing paths to allow the phase angles at the modulation frequencyassociated with each resonant circuit to be measured in parallel.Alternatively, the resonant circuits could be alternately energised andthe resultant signal induced in the sensor winding input into a singleprocessing path in which the frequency of the synchronous detection isalternated in accordance with the energised resonant circuit.

In the illustrated embodiment, the excitation signal generating andsensed signal processing circuitry employs the general principlesdisclosed in GB 2374424A. However, alternative forms of excitationsignal generating and sensed signal processing could be employed. Forexample, instead of having two excitation windings in the transmitaerial and detecting the phase of a signal induced in receive aerialformed by a single sensor winding, the transmit aerial could be formedby a single excitation winding and the receive aerial could be formed bytwo sensor windings, with the coupling between the excitation windingand the two sensor windings varying with rotary position. The generalprinciples of such a rotary encoder are discussed in WO 95/31696.

In the described embodiment, carrier frequencies from 1.875 MHz to 5 MHzare used. It will be appreciated that the exact values of the carrierfrequencies (and accordingly the resonant frequencies of the resonantcircuits) is a design choice, although preferably the carrierfrequencies are in the range 100 kHz to 10 MHz to achieve good signalcoupling with comparatively cheap excitation and synchronous detectioncircuitry. The modulation frequency is also a design choice.

In the illustrated embodiment, the two sensing arrangements have aperiodicity of twenty periods over 360° and nineteen periods over 360°respectively to enable absolute position measurement be carried out. Itwill be appreciated that alternative periodicities could be used.Further, if absolute position measurement was not required then theperiodicities for the first and second sensing arrangements could beidentical.

While in the illustrated embodiment absolute position measurement isobtained only with reference to the housing, it will be appreciated thatthe absolute position within the entire range of rotary movement couldbe measured by either employing an extra sensor to count revolutions orby continuously monitoring the rotary position in order to keep track ofthe revolutions.

Although separate ASICs are used for the two sensing arrangements in theillustrated embodiment for safety reasons, this is not essential and inmany applications a common ASIC could be used for both sensingarrangements while still satisfying safety requirements. In someapplications, the redundant sensing arrangement is not necessary and theredundant sensing arrangement accordingly need not be included.

In the illustrated embodiment, the output of each ASIC is representativeof the relative rotary displacement between the first and second shaftsand the central control unit determines the applied torque. It will beappreciated that the ASIC could perform linearisation and/or calibrationprocessing. In an alternative embodiment, the ASIC could determine theapplied torque. In another alternative embodiment, the output of eachASIC is representative of the detected phase angles associated with thefirst and second shafts, and the central control unit calculates boththe relative rotary displacement and the applied torque.

It will be appreciated that there are many conventional signallingsystems which could be used to transfer data between the ASICs and thecentral control unit, e.g. pulse width modulation or pulse codemodulation.

Although ASICs are used in the illustrated embodiment, this is notessential and any other processing means could be employed, includingusing discrete electronic components.

In the illustrated embodiment, the aerials and resonant circuits areformed using PCB technology. This is not essential and other techniquesfor arranging conductive tracks, including arranging wire tracks, couldalternatively be used.

In the illustrated embodiment, the torque sensor measures the torqueapplied to a steering wheel of a car. It will be appreciated that thereare many other places in a car where a torque is applied and theinductive sensor according to the invention could be used. For example,the torque applied to a drive shaft could be measured. Further, theinductive torque sensor of the present invention also has applicationoutside of the automotive industry. For example, the inductive sensor ofthe present invention could be used to measure the torque applied to adrill.

In the illustrated embodiment, a torsion bar arrangement is used inwhich two bars are fixed to each other, and relative rotary movementbetween the two bars is measured. This is generally advantageous inapplications where the torsion bar must be made of a stiff material.However, in other applications a less stiff material may be acceptable,in which case the twisting of a single bar could be measured. In otherwords, the relative rotary displacement between two axial positions ofthe same member could be measured.

1. Apparatus for generating rotary displacement informationrepresentative of the torsion applied to a torsion bar which isrotatable, relative to a housing, about an axis of rotation, theapparatus comprising: a first resonator fixed relative to a first axialposition of the torsion bar, the first resonator having a first resonantfrequency; a second resonator fixed relative to a second axial positionof the torsion bar which is away from the first axial position, thesecond resonator having a second resonant frequency which is differentfrom the first resonant frequency; a transmit aerial fixed relative tothe housing; and a receive aerial fixed relative to the housing, whereinat least one of the electromagnetic coupling between the transmit aerialand the first resonator and the electromagnetic coupling between thefirst resonator and the receive aerial varies with the rotary positionof the torsion bar relative to the housing at said first axial positionso that a signal induced in the receive aerial by a first resonantsignal induced in the first resonator by an electromagnetic fieldproduced by the transmit aerial varies with the rotary position of thetorsion bar relative to the housing at said first axial position, andwherein at least one of the electromagnetic coupling between thetransmit aerial and the second resonator and the electromagneticcoupling between the second resonator and the receive aerial varies withthe rotary position of the torsion bar relative to the housing at saidsecond axial position so that a signal induced in the receive aerial bya second resonant signal induced in the second resonator by anelectromagnetic field produced by the transmit aerial varies with therotary position of the torsion bar relative to the housing at saidsecond axial position.
 2. Apparatus according to claim 1, wherein thetransmit aerial is operable to generate an electromagnetic field whichvaries periodically along a rotary measurement path, and wherein atleast one of the first and second resonators comprises two or morecurrent loops angularly spaced in accordance with an integer multiple ofthe angular period of the transmit aerial.
 3. Apparatus according toclaim 1, wherein the transmit aerial, the receive aerial and the firstand second resonators are formed by conductive windings arranged axiallyand circumferentially around the torsion bar.
 4. Apparatus according toclaim 3, wherein at least one of the transmit aerial, the receive aerialand the first and second resonators are formed on a flexible printedcircuit board.
 5. Apparatus according to claim 1, wherein said two ormore current loops are circumferentially arranged with current loopsgenerally facing each other from opposing sides of the torsion bar. 6.Apparatus according to claim 1, wherein the first resonator and thesecond resonator comprise current loops formed in circumferentiallyspaced portions of a common axial position.
 7. Apparatus according toclaim 1, wherein the first resonator and the second resonator areaxially spaced from each other.
 8. Apparatus according to claim 1,wherein the transmit aerial, the receive aerial and the first and secondresonators are formed by conductive windings arranged radially andcircumferentially around the torsion bar.
 9. Apparatus according toclaim 1, wherein said transmit aerial is a first transmit aerial andsaid receive aerial is a first receive aerial, and wherein the apparatusfurther comprises: a third resonator fixed relative to a third axialposition of the torsion bar, the third resonator having a third resonantfrequency different from the first and second resonant frequencies; afourth resonator fixed relative to a fourth axial position of thetorsion bar which is spaced away from said third axial position, thefourth resonator having a fourth resonant frequency which is differentfrom the first, second and third resonant frequencies; a second transmitaerial fixed relative to the housing; and a second receive aerial fixedrelative to the housing, wherein at least one of the electromagneticcoupling between the second transmit aerial and the third resonator andthe electromagnetic coupling between the third resonator and the receiveaerial varies with the rotary position of the torsion bar relative tothe housing at said third axial position so that a signal induced in thereceive aerial by a third resonant signal induced in the third resonatorby an electromagnetic field produced by the second transmit aerialvaries with the rotary position of the torsion bar relative to thehousing at said third axial position, and wherein at least one of theelectromagnetic coupling between the transmit aerial and the fourthresonator and the electromagnetic coupling between the fourth resonatorand the receive aerial varies with the rotary position of the torsionbar relative to the housing at the fourth axial position so that asignal induced in the receive aerial by a fourth resonant signal inducedin the fourth resonator by an electromagnetic field produced by thesecond transmit aerial varies with the rotary position of the torsionbar relative to the housing at the fourth axial position.
 10. Apparatusaccording to claim 9, wherein the first and third resonators are formedby current loops in common circumferential portions relative to thetorsion bar, with the current loops of the first resonator being axiallyspaced from the current loops of the third resonator, wherein the secondand fourth resonators are formed by current loops in commoncircumferential portions relative to the torsion bar, with the currentloops of the second resonator being axially spaced from the currentloops of the fourth resonator.
 11. Apparatus according to claim 10,wherein the first and third resonators are formed on extended regions ofa first castellated member fixed to the torsion bar, wherein the secondand fourth resonators are formed on extended regions of a secondcastellated member fixed to the torsion bar, and wherein the first andsecond castellated members interlock with each other while allowing atleast some relative rotary displacement.
 12. Apparatus according toclaim 9, wherein the electromagnetic coupling between the first aerialand the first receive aerial via the first and second resonators varieswith a first angular periodicity, and the electromagnetic couplingbetween the second transmit aerial and the second receive aerial via thethird and fourth resonators varies with a second angular periodicitywhich is different from the first angular periodicity.
 13. Apparatusaccording to claim 1, wherein the torsion bar comprises a first elongateshaft and a second elongate shaft whose longitudinal axes are alignedwith the axis of rotation, wherein the first and second shafts arecoupled at an axial position away from said first and second axialpositions.
 14. Apparatus according to claim 1, further comprising anexcitation signal generator for supplying an excitation signal to thetransmit aerial and a signal processor for processing the signal inducedin the receive aerial.
 15. Apparatus according to claim 14, wherein theexcitation signal generator is operable to energise the first and secondresonators simultaneously.
 16. Apparatus according to claim 14, whereinthe signal processor is operable to process signals induced in thereceive aerial by the first resonator and the second resonatorsimultaneously.
 17. Apparatus according to claim 14, wherein theexcitation signal generator is operable to energise the first and secondresonators alternately.
 18. Apparatus according to claim 14, wherein thesignal processor is operable to process alternately signals induced inthe receive aerial by the first resonator and the second resonator. 19.Apparatus according to claim 14, wherein the signal processor isoperable to generate a signal conveying information representative ofthe relative rotary displacement between said first and second axialpositions of the torsion bar.
 20. Apparatus according to claim 14,wherein the signal generator is operable to generate an excitationsignal comprising a periodic carrier signal having a first frequencymodulated by a periodic modulation signal having a second frequency, thefirst frequency being greater than the second frequency.
 21. Apparatusaccording to claim 20, wherein the signal processor comprises ademodulator operable to demodulate the induced signal generated in thereceive aerial to obtain a demodulated signal at the second frequency.22. Apparatus according to claim 21, wherein the signal processorfurther comprises a phase detector operable to detect the phase of thedemodulated signal at the second frequency.
 23. An apparatus forproviding rotary displacement information indicative of the torqueapplied to a torsion bar which is rotatable relative to a housing, inwhich the electromagnetic coupling between a transmit aerial fixedrelative to the housing and a receive aerial fixed relative to thehousing varies in dependence upon the rotary position of first andsecond resonators fixed relative to spaced axial positions of thetorsion bar, wherein the first and second resonators have respectivedifferent resonant frequencies to enable the signal coupled between thetransmit aerial and the receive aerial via the first resonator to bedistinguished from the signal coupled between the transmit aerial andthe receive aerial via the second resonator.